Derive macros for RTC Interceptor trait.

This crate provides two macros that work together:

• #[derive(Interceptor)] - Marks a struct as an interceptor and identifies the next field
• #[interceptor] - Attribute macro for impl blocks to generate trait implementations

Design Pattern

The design follows Rust's derive pattern (similar to #[derive(Default)] with #[default]):

use rtc_interceptor::{Interceptor, interceptor, TaggedPacket, Packet, StreamInfo};
use std::collections::VecDeque;

#[derive(Interceptor)]
pub struct MyInterceptor<P: Interceptor> {
    #[next]
    next: P,  // The next interceptor in the chain (can use any field name)
    buffer: VecDeque<TaggedPacket>,
}

#[interceptor]
impl<P: Interceptor> MyInterceptor<P> {
    #[overrides]
    fn handle_read(&mut self, msg: TaggedPacket) -> Result<(), Self::Error> {
        // Custom logic here
        self.next.handle_read(msg)
    }
}

Pure Delegation (No Custom Logic)

For interceptors that just pass through without modification:

#[derive(Interceptor)]
pub struct PassthroughInterceptor<P: Interceptor> {
    #[next]
    next: P,
}

#[interceptor]
impl<P: Interceptor> PassthroughInterceptor<P> {}
// Empty impl block - all methods are auto-generated

Required Imports

The macros require certain types to be in scope:

use rtc_interceptor::{Interceptor, interceptor, TaggedPacket, Packet, StreamInfo};
// Or through rtc umbrella crate:
use rtc::interceptor::{Interceptor, interceptor, TaggedPacket, Packet, StreamInfo};
use rtc::shared::error::Error;
use rtc::sansio;  // Required for macro-generated code